Tritium is one of the most environmentally significant radioisotopes emitted from nuclear fuel reprocessing plants and waste solidification facilities. Tritium is difficult to measure in real time due to its weak beta radiation, its many chemical forms (HT, HTO, CH.sub.3 T) and the presence of other radioactive and nonradioactive constituents in the off-gas stream. For example, the off-gas stream from a typical waste solidification facility contains oxides of nitrogen, particulate activities, water vapors, .sup.14 C and .sup.85 Kr. In particular, the .sup.14 C and .sup.85 Kr have presences that are difficult to minimize in order to prevent these elements from biasing, the tritium measurement.
The need for tritium monitoring of effluents arises primarily from tritium generated in the coolant by neutron activation of deuterium. Ternary fission in the fuel and activation of impurities in the water are very minor contributors to the tritium inventory in the heavy water (D.sub.2 O) coolant. Tritium is generally present in heavy water as tritiated water (DTO), HTO, or T.sub.2 O and follows the water pathway of the D.sub.2 O such as that occurring from leaks, vaporization, and water adhering to surfaces of fuel rods and equipment removed during refueling. Tritium concentrations in the moderatorcoolant may range from several hundred u Ci/ml at the National Bureau of Standards test reactor to 10,000-20,000 .mu.Ci/ml in production reactors.
The purpose of monitoring can be resolved into three categories: (1) to measure the total release of tritium to the environment, assuring that emissions are within acceptable levels, (2) to measure the rate of release, which provides diagnostic information about the general operating performance of the plant, and (3) to prevent personnel exposure.
In the past, one manner of analyzing the tritium levels involved the conversion by catalytic oxidation, or other manner, of all forms of tritium to HTO, separating the HTO from the .sup.85 Kr and .sup.14 C and in measuring the tritium in a purified sample stream. Other means employed silica gel beds and molecular sieves. In general, the analysis for tritium in the effluent of a nuclear fuel reprocessing plant required that samples be collected and chemical separation and liquid scintillation analysis performed in the laboratory.
Accordingly, an object of the subject invention is the on-line continuous monitoring and analysis of tritium levels from the liquid effluent of a nuclear fuel reprocessing plant.
A further object of the subject invention is a means for analyzing tritium in a liquid effluent through the use of a semi-permeable membrane.
A still further object of the subject invention is a means for on-line continuous analysis of tritium concentrations from 1 to 1,000 u Ci/M.sup.3 in the presence of other radioactive materials of varying concentration and identity.
These and other objects are obtained in accordance with the subject invention wherein there is provided a method and apparatus for the determination of aqueous tritium concentration on a continuous on-line basis. A liquid sample enters the monitoring apparatus from a pressurized process line through a filter and a metering valve. The liquid sample is passed against an air stream in a packed humidifier column to form a moist air stream. The saturated air stream leaving the humidifier is directed through a heated transfer line to a permeation dryer employing a semi-permeable membrane where the tritium content in the form of HTO, T.sub.2 O, and DTO is preferentially transferred to a second air stream along with water vapor. As used herein, the term tritium material will refer to HTO, T.sub.2 O, and DTO and mixtures of these substances. The air stream, now containing only tritium material flows past a relative humidity/temperature transducer and then through a flowthrough ionization chamber. The ion chamber response, relative humidity, and temperature parameters are communicated to a process-control computer for calculation of the tritium content of the liquid. The calculation involves a determination of the partial pressure of the H.sub.2 O in the ion chamber followed by substitution into a form of the ideal gas law to obtain the tritium concentration per unit mass of H.sub.2 O.